As representative safety equipment for trains to safely run and stop, automatic train stop (ATS) and automatic train control (ATC) are known. Moreover, as representative operation support equipment, automatic train operation (ATO) and train automatic stop-position controller (TASC) are known.
The conventional safety equipment and operation support equipment perform train-body position detection using a transponder, track circuitry, etc., and have a problem that it takes much cost to install the transponder, track circuitry, etc.
In view of the above circumstances, it is considered to use RFID (Radio Frequency Identification) for position detection. The RFID is a mechanism in which a reader wirelessly reads individual identification information stored in a tag. The RFID can be implemented with a much lower cost than position detection using the transponder, track circuitry, etc., and with a relatively low maintenance cost.
In the case of train-body position detection using the RFID, it is desirable to install a reader at a body side and a tag at a ground side. Because, if the tag at the ground side is a passive type with no power needed, auxiliary facilities to be installed along with tag installation are not needed and hence a facility cost at the ground side can be restricted.
Wireless communication with the tag and the reader becomes more stable as the distance between the tag and the reader is shorter. However, as the distance between the tag and the reader is shorter, the wireless communication range becomes smaller, so that skip reading may occur while a train is running at high speeds. Conversely, as the distance between the tag and the reader is longer, since the wireless communication range becomes larger, the frequency of skip reading is reduced while the train is running at high speeds, however, position accuracy is lowered while the train is running at low speeds.
As described above, there are an advantage and a disadvantage for both cases of a long distance and a short distance between the tag and the reader. It is therefore desirable, if possible, to install tags in a plurality of locations with different distances from the reader.
Since a train on-board system demands high reliability, it is required to be a duplex system under consideration of occurrence of failure in wireless communication for some reasons. As described above, in the case of installing tags on a track and along the track, and if the duplex system is implemented in each of the tags on the track and along the track, the number of antennas is increased, resulting in a complicated reader configuration.
In the case of a single antenna having directivity in different directions, by making larger the size of the antenna element than the wavelength of a used frequency, the antenna can have bidirectional directivity. However, in the case of RFID in a 920-MHz band, the wavelength is about 33 cm, it is highly likely that the size of an antenna including the outer frame is larger than the size of the conventional transponder on-board equipment. The duplex system requires two antenna elements of this size and hence there is a difficulty in securing an installation space in the train body.
In the case where a tag is installed in the center area of a track, if a reader antenna is installed on the bottom of a train body just above the tag, since there are several kinds of under-floor equipment, such as a motor, under the bottom of the train body, electromagnetic waves from the antenna are reflected by the under-floor equipment, causing unstable wireless communication with the tag.